The first objective of this proposal is to determine the manner in which muscles and populations of fibers within muscles are recruited and utilized during locomotion as animals increase speed and change gaits. We plan to re-design a 'force platform system' which we will use together with high speed films and anatomical dissections to measure muscle stresses (force/cross-sectional area) in limb extensor muscle groups during the time of foot contact. In the first year we plan a direct comparison of muscle stresses determined using this force platform system with values obtained using 'force buckles' on tendons in order to quantitatively evaluate errors of both techniques. The measurements of muscle forces and stresses will be used to pursue our specific goals: 1) To determine how peak forces developed by limb muscles and the time course of force development within a stride change to enable animals to sustain different speeds within a gait (walk, trot, gallop); 2) To determine whether peak muscle stresses are reduced when animals change gaits as they increase speed (walk to trot/run; trot to gallop); and whether they reach similar values at the highest trotting/running and highest galloping speeds; and 3) To determine whether peak muscle stresses during locomotion are similar under equivalent conditions in different mammalian species (e.g. walk-trot transition, trot-gallop transition, top speed, maximum acceleration and maximum deceleration). The second objective of this proposal is to investigate the relationship between muscular activity and the upper and lower limits of metabolism. We have found that there is not a tight link between metabolism averaged over 24-hour intervals and either resting or maximal rates of oxygen consumption. However, we have found that the maximum rate of oxygen consumption observed during spontaneous activity is approximately half of maximum aerobic capacity in all three species. We plan to extend these studies to include additional species where maximum oxygen consumption during spontaneous activity varies by 2-fold in order to test and extend the observation that the aerobic capacity of animals appears related to, and perhaps maintained by, a short period of spontaneous exercise each day at about 50% of maximum aerobic capacity.